In the production of packages and electronic devices, a semiconductor or micromechanical die will typically be soldered to a package substrate. The package substrate is typically soldered to a printed circuit board, such as a motherboard, daughter board or some other board. In these soldering operations, the entire die and substrate are heated to a solder reflow temperature. In later processing steps, parts may be heated to even higher temperatures. The heat may cause the die, the substrate, the package, or even a printed circuit board to warp. The warping is caused by a difference in coefficient of thermal expansion between one part of a component and another.
The parts that are soldered together have arrays of contacts that line up when the two parts are placed against each other. If one part warps more or less than another part, then the contacts do not line up and some contacts will be farther apart than other contacts. This puts stress on the solder that connects the contacts and can cause the solder connection to be broken or never made.